In recent years, broadband network architectures for CATV networks have evolved from unidirectional analog systems to bi-directional, Hybrid Fiber Coaxial (HFC) systems with a mix of analog and digital signals. Such networks may deliver analog/digital video, analog/digital audio, and high speed data to cable subscribers. The most common configuration comprises a fiber optic main distribution network associated with a local distribution network using coaxial cable. For traditional broadcast TV service, most HFC CATV networks collect satellite and trunk cable feeds, local off-the-air television channels, and other video/audio channels and distribute them from the headend using an analog modulated signal scheme such as an amplitude modulated vestigial sideband (AM-VSB) scheme. The channels are placed onto different RF sub-carriers within a frequency spectrum allocated for CATV downstream transmission (typically 50 to 550 MHz), with each channel generally occupying 6 MHz of the spectrum. On the other hand, most new services being offered on CATV networks such as video-on-demand (VOD), digital TV, high-speed data (HSD), and IP telephony, are distributed using digital modulated RF sub-carriers. The digital modulated signals are typically multilevel quadrature amplitude modulated (M-QAM) sub-carriers within an RF band that is often between about 550-870 MHz. In the M-QAM scheme, both the amplitude and phase of the sub-carrier are varied to represent each digital symbol. For example, in a 256 QAM, 256 combinations of amplitude and phase are used. Finally, the M-QAM RF sub-carriers and the AM-VSB RF sub-carriers may be combined so that the resulting frequency multiplexed subcarrier signal may be used to modulate an optical carrier generated by a laser. This modulation and multiplexing scheme is sometimes referred to as a hybrid multichannel AM-VSB/M-QAM transport architecture.
When a number of frequency multiplexed subcarriers are simultaneously optically transmitted in a CATV network, the resulting modulated optical carrier signal may experience clipping distortion. The phenomena of clipping is illustrated in the graph of FIG. 1, which is a plot of a laser diode optical output L as a function of an the laser drive current I. FIG. 1 also shows a typical transfer characteristic (curve B) for the laser diode. The laser threshold point is also indicated on this plot. An applied RF input signal is impressed as shown on the laser drive current I. As shown, the swing of this drive signal is large enough to drive the laser below its threshold current, denoted as point “A” in FIG. 1. Driving the laser in this fashion results in the laser output being clipped, illustrated as region C. That is, the optical output signal of the laser diode exhibits clipped or flattened negative-going peaks in region C because of the transitions of the laser drive current below the threshold current.
Clipping can be a particular problem when analog subcarrier signals using vestigial sideband amplitude modulation (VSB-AM) are transmitted because it is generally important to set a high optical modulation index to ensure a large carrier-to-noise ratio (CNR). Because a large number of signals are being combined, occasional distortion of this type is almost inevitable. It may occur, for example, when multiple superimposed sub-carriers at different frequencies momentarily re-enforce each other to produce a drive current pulse that exhibits a relatively large displacement from the laser's bias current Ibias. When clipping does occur, the resulting momentary distortion caused by the pulse can greatly deteriorate the transmission quality (as measured in terms of the Bit Error Rate (BER), for example) of the digital subcarrier signal.
Another problem caused by clipping is that it effectively prevents the modulation index of the subcarrier signals from being increased to thereby increase the carrier to noise ratio (CNR) since such an increase would cause additional clipping to occur.
Various techniques are known for reducing clipping, particularly for directly modulated optical transmitters. For instance, pre-clipping, pre-distortion and dissymmetrization techniques may be employed. However, these techniques are difficult if not impossible to use with optical transmitters that are externally modulated.